Heat treating and inspection of metals Page: 3
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HEAT TREATING AND INSPECTION OF METALS 1
material. In general, both ferrous and nonferrous metals as well
as their alloys respond to some form of heat treatment and this fact
is made use of to a great extent in the manufacture of aircraft parts
and assemblies. Most all metals have a critical temperature at which
the grain structure changes. Successful heat treatment, therefore,
depends largely on a knowledge of these temperatures as well as the
time required to produce the desired change.
b. Heat treatment involves a cycle of events which may be described
(1) Heating.-Heating a metal to a temperature within or above
its critical temperature under carefully controlled conditions.
(2) Soaking or holding.-Keeping a metal at an elevated tempera-
ture for a definite time, in order that it may become thoroughly
saturated with heat, and permit the necessary changes in grain struc-
ture to take place.
(3) Cooling.-Returning a metal to room temperature by quench-
ing in air, brine, water, oil, etc.
c. The most common forms of heat treatment for ferrous metals
are: annealing, normalizing, hardening, tempering, and case
(1) Annealing is used to reduce stresses, induce softness, alter
ductility, or refine the grain structure. Maximum softness in metal
is accomplished by heating it to a point above the critical tempera-
ture, holding at this temperature until the grain structure has been
refined, followed by slow cooling.
(2) Normalizing is a process whereby iron base metals are heated
above their critical temperature to obtain better solubility of the
carbon in the iron, followed by cooling in still air.
(3) Hardening is accomplished by heating the metal slightly in
excess of the critical temperature, then rapidly cooling by quenching
in oil, water, or brine. This treatment produces a fine grain struc-
ture, extreme hardness, maximum tensile strength, and minimum
ductility. Generally, material in this condition is too brittle for
most practical uses, although this treatment is the. first step in the
production of high strength steel.
(4) Tempering (drawing) is a process generally applied to steel
to relieve the strains induced during the hardening process. It con-
sists of heating the hardened steel to a temperature below the criti-
cal range, holding this temperature for a sufficient pe-;od, then cool-
ing in water, oil, or air. In this process, the degrees of strength,
hardness, and ductility obtained, depends directly upon the tempera-
ture to which the steel is heated.
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/5/: accessed March 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.